1. Field of the Invention
The present invention relates to a plasma etching method and apparatus for locally etching convex portions on a surface of an object to be treated.
2. Description of the Related Art
For a surface etching technology for etching a surface of an object such as a silicon wafer, there have been proposed a variety of techniques in which an object to be etched is exposed to a plasma-excited active species gas atmosphere so as to grind and polish the entire surface of the object, or in which an object to be etched is partially masked with a non-masked portion thereof being etched by means of an active species gas to form a circuit pattern.
In recent years, in place of the technique of etching the entire surface of an object, new technologies such as a TTV (Total Thickness Variation) technique, an LTV (Local Thickness Variation) technique and the like have been proposed in which convexes on a surface of an object to be etched such as a silicon wafer, a silicon-on-insulator device (SOI) and the like are subjected to localized etching, to thereby thin the etched object, or flatten the surface to improve variations in shaping or configuration of the object (for example, see Japanese Patent Laid-Open No. 6-5571).
FIG. 14 schematically shows the principle of such conventional techniques. In this figure, a reference numeral 100 designates a plasma generator which generates a plasma gas containing an active species gas G which is injected to a surface of an object to be etched in the form of a wafer 110 by means of a nozzle 101 through its opening 102.
The wafer 110 is disposed on and fixed to a stage 120 so that the stage 120 can be moved in horizontal directions to guide a convex portion 111 of the wafer 110 into a position just under the opening 102 of the nozzle 101. Then, the active species gas G is ejected to the convex portion 111 of the wafer 110 to locally etching the convex portion 111 to thereby flatten the surface of the wafer 110.
In the above-mentioned conventional techniques, however, there arise the following problems. The sizes or dimensions of respective convexes of the convex portion 111 are varying, that is, for example with a silicon wafer having a diameter of 8 inches, there is a first one having an angle-shaped configuration with a highest or thickest near-center portion and a lower peripheral portion, a second one having a cone-shaped bottom configuration with a highest or thickest peripheral portion and a lowest or thinnest central portion, a third one having a multitude of small convexes and concaves each having a diameter of less than several millimeters, and a fourth one of mixed type having a mixed configuration with at least two of the above types being superposed or mixed with each other. In this manner, the convexes on the surface of the wafer are not uniform in size or dimensions thereof and not of the single type, but varying in size and type thereof.
On the other hand, since the active species gas is ejected from the nozzle 101, the diameter D of the opening 102 of the nozzle 101 is substantially the same as the diameter of an area to be etched of the wafer 110, so that the area of the wafer 110 is uniformly etched by means of the active species gas G. Accordingly, in the case where the wafer 110 has a multitude of convexes 111 of varying diameters on a surface thereof, the diameter D of the nozzle opening 102 has to be set so as to meet the diameter of the smallest one of the convexes, as shown in FIG. 15. This is because if the diameter D of the nozzle opening 102 is set to a value corresponding to that of a larger convex 111b, concaves 112 near and around small convexes 111a are to be etched when etching the small convexes 111a. However, with the technique in which the diameter D of the nozzle opening 102 is matched to that of the smallest convex 111a, upon etching a larger convex 111b, a number of (i.e., from several to tens) etching treatments are required, thus prolonging the time necessary for one surface flattening operation.
For this reason, in order to carry out such a surface flattening operation in a short period of time using the above-mentioned conventional technique, it is generally required that a plurality (e.g., two in the illustrated example) of plasma generators 100-1, 100-2 having different diameters of nozzle openings be provided for respective treatment chambers A, B so as to etch the wafer 110 by means of the plasma generators 100-1, 100-2 in sequence. For example, the treatment chamber A is constructed such that it is equipped with the plasma generator 100-1 having a nozzle 101 with its opening of 30 mm, and the treatment chamber B is constructed such that it is equipped with the plasma generator 100-2 having a nozzle 101 with its opening of 7 mm. A wafer 110 is first supplied to the treatment chamber A in which relatively large convexes 111b on a surface of the wafer 110 each having a diameter equal to or greater than 30 mm are subjected to plasma etching. The wafer 110 thus treated is then transported to the treatment chamber B in which convexes 111a each having a diameter less than 30 mm are plasma etched.
With such a technique, however, the surface flattening time is in fact shortened, but at least two treatment chambers A, B equipped with the plasma generators 100-1, 100-2 are required, resulting in a substantial increase in the cost of equipment. Moreover, the wafer 110 has to be transported from the treatment chamber A to the treatment chamber B. thus prolonging the time of the entire etching treatments required.